Expression Of Conditioned Place Aversion Research Articles

Cannabidiol attenuates the expression of conditioned place aversion induced by naloxone-precipitated morphine withdrawal through the activation of 5-HT1A receptors

The misuse of and addiction to opioids are serious public health problems in some countries, such as the USA. Drug addiction is a chronic and relapsing medical condition that involves motivational and memory-related processes due to the strong associations between drugs and consuming-related stimuli. These stimuli usually trigger continuous and compulsive use and are associated with relapses after periods of withdrawal. Several factors contribute to relapse, including withdrawal-induced mood changes. Therefore, drugs attenuating withdrawal-induced affective alterations could be useful alternative treatments for relapse prevention. Cannabidiol (CBD), a non-psychotomimetic component from the Cannabis sativa plant, has anti-anxiety and anti-stress properties and has been investigated as an alternative for the treatment of several mental disorders, including drug addiction. Here, we evaluated if CBD administered 30 min prior to test for a conditioned place aversion (CPA) would attenuate the aversion induced by morphine withdrawal precipitated by the opioid receptor antagonist naloxone in male C57BL/6 mice. We also investigated if this effect involves the activation of 5-HT1A receptors, a mechanism previously associated with CBD anti-aversive effects. As expected, morphine-treated mice spent less time exploring the compartment paired with the naloxone-induced withdrawal, indicating a CPA induced by naloxone-precipitated morphine withdrawal. This effect was not observed in animals treated with CBD, at 30 and 60 mg/kg, prior to the CPA test, indicating that CBD attenuated the expression of CPA induced by naloxone-precipitated morphine withdrawal. Pretreatment with the 5-HT1A receptor antagonist WAY100635 (0.3 mg/kg) blocked CBD effects. Our findings suggest that CBD may reduce the expression of a previously established conditioned aversion induced by morphine withdrawal by a mechanism involving the activation of 5-HT1A receptors. Thus, CBD may be a therapeutic alternative for preventing relapse to opioid addiction by decreasing withdrawal-induced negative affective changes.

Naloxone-induced conditioned place aversion score and extinction period are higher in C57BL/6J morphine-dependent mice than in Swiss: Role of HPA axis

Intense associative memories develop between drug-paired contextual cues and the drug withdrawal associated aversive feeling. They have been suggested to contribute to the high rate of relapse. Our study was aimed to elucidate the involvement of hypothalamic-pituitary-adrenocortical (HPA) axis activity in the expression and extinction of aversive memory in Swiss and C57BL/6J (B6) mice. The animals were rendered dependent on morphine by i.p. injection of increasing doses of morphine (10–60 mg/kg). The negative state associated with naloxone (1 mg/kg s.c.) precipitated morphine withdrawal was examined by using conditioned place aversion (CPA) paradigm. B6 mice obtained a higher aversion score and took longer to extinguish the aversive memory than Swiss mice. In addition, corticosterone levels were increased after CPA expression. Moreover, corticosterone levels were decreased during CPA extinction in Swiss mice without changes in B6 mice. Pre-treatment with the selective CRF1 receptor antagonist CP-154,526 before naloxone, impaired morphine-withdrawal aversive memory acquisition and decreased the extinction period. CP-154,526 also antagonized the increased levels of corticosterone observed after CPA expression in Swiss mice, without any changes in B6 mice. These results indicate that HPA axis could be a critical factor governing opioid withdrawal memory storage and retrieval, but in a strain or stock-specific manner. The differences observed between Swiss and B6 mice suggest that the treatment of addictive disorders should consider different individual predisposition to associate the aversive learning with the context.

Morphine and ethanol pretreatment effects on expression and extinction of ethanol-induced conditioned place preference and aversion in mice.

Opioid receptor antagonists reliably alter the expression or extinction of ethanol's conditioned motivational effects as indexed by the place conditioning procedure, suggesting endogenous opioids are normally involved. These studies examined how exogenous stimulation of opioid receptors alters ethanol's conditioned rewarding and aversive effects. Drugs that either directly (morphine) or indirectly (ethanol) stimulate opioid receptors were tested for their effects on the expression and extinction of ethanol-induced conditioned place preference (CPP) and conditioned place aversion (CPA). Male DBA/2J mice were exposed to unbiased ethanol (2 g/kg) conditioning procedures that produced either CPP (experiments 1-2) or CPA (experiments 3-4). Morphine (0, 2.5, 5, or 10 mg/kg) was injected before three post-conditioning tests in experiments 1 and 3, whereas ethanol (0, 1, 2, or 3 g/kg) was injected before tests in experiments 2 and 4. All groups received vehicle on test 4 to determine whether the drug pretreatments altered the course of extinction. Morphine dose-dependently enhanced CPP expression (experiment 1), but ethanol dose-dependently reduced CPP expression (experiment 2). Test 4 showed no differences between drug-treated mice and mice given vehicle on all tests. Morphine had no effect on expression or extinction of ethanol-induced CPA (experiment 3). The highest ethanol dose (3 g/kg) interfered with CPA expression, but not extinction (experiment 4). Pretreatment drug effects on ethanol CPP and CPA expression were most likely a byproduct of their activity altering effects rather than opioid-receptor mediated modulation of ethanol's conditioned motivational effects. Neither drug affected the course of extinction.

Liposome-Encapsulated Morphine Affords a Prolonged Analgesia While Facilitating Extinction of Reward and Aversive Memories.

Morphine is thoroughly used for pain control; however, it has a high addictive potential. Opioid liposome formulations produce controlled drug release and have been thoroughly tested for pain treatment although their role in addiction is still unknown. This study investigated the effects of free morphine and morphine encapsulated in unilamellar and multilamellar liposomes on antinociception and on the expression and extinction of the positive and negative memories associated with environmental cues. The hot plate test was used to measure central pain. The rewarding effects of morphine were analyzed by the conditioned-place preference (CPP) test, and the aversive aspects of naloxone-precipitated morphine withdrawal were evaluated by the conditioned-place aversion (CPA) paradigm. Our results show that encapsulated morphine yields prolonged antinociceptive effects compared with the free form, and that CPP and CPA expression were similar in the free- or encapsulated-morphine groups. However, we demonstrate, for the first time, that morphine encapsulation reduces the duration of reward and aversive memories, suggesting that this technological process could transform morphine into a potentially less addictive drug. Morphine encapsulation in liposomes could represent a pharmacological approach for enhancing extinction, which might lead to effective clinical treatments in drug addiction with fewer side effects.

Conditioned aversive memory associated with morphine withdrawal increases brain-derived neurotrophic factor in dentate gyrus and basolateral amygdala.

Morphine has been shown to increase the expression of brain-derived neurotrophic factor (BDNF) in the brain. However, little is known about the effect of conditioned naloxone-precipitated morphine withdrawal on BDNF and its precursor protein, proBDNF. We used the conditioned place aversion (CPA) paradigm to evaluate the role of corticotropin-releasing factor (CRF)/CRF1 receptor signaling on the BDNF expression and corticosterone plasma levels after CPA expression and extinction. Male mice were rendered dependent on morphine and injected acutely with naloxone before paired to confinement in a naloxone-associated compartment. The expression of BDNF and proBDNF in the dentate gyrus (DG) and basolateral amygdala (BLA) was measured in parallel with the corticosterone plasma levels with and without CRF1 receptor blockade. Mice subjected to conditioned naloxone-induced morphine withdrawal showed an increased expression of BDNF (in DG and BLA) in parallel with an enhancement of corticosterone plasma levels. These results demonstrated that BDNF expression together with the increased activity of hypothalamic-pituitary-adrenocortical (HPA) axis are critical to the acquisition of aversive memory. However, we have observed a decrease in corticosterone plasma levels and BDNF expression after CPA extinction reaffirming the importance of BDNF in the maintenance of aversive memory. In addition, the pre-treatment with the CRF1 receptor antagonist CP-154526 before naloxone conditioning session impaired morphine withdrawal-induced aversive memory acquisition, the increased corticosterone plasma levels, and the expression of BDNF observed after CPA expression in the DG and BLA. Altogether, present results are suggesting a clear connection between HPA axis and BDNF in the formation and extinction of aversive memory.

Regulation and function of MeCP2 Ser421 phosphorylation in U50488-induced conditioned place aversion in mice.

Phosphorylation of the methyl DNA-binding protein MeCP2 at Ser421 (pMeCP2-S421) is induced in corticolimbic brain regions during exposure to drugs of abuse and modulates reward-driven behaviors. However, whether pMeCP2-S421 is also involved in behavioral adaptations to aversive drugs is unknown. Our goal was to establish the role and regulationof pMeCP2-S421 in corticolimbic brain regions of mice upon acute treatment with the kappa opioid receptor agonist U50488 and during the expression of U50488-induced conditioned place aversion (CPA). pMeCP2-S421 levels were measured in the nucleus accumbens (NAc), prelimbic cortex, infralimbic cortex (ILC), and basolateral amygdala (BLA) of male mice after intraperitoneal administration of U50488 and upon the expression of U50488-induced CPA. Fos was measured as marker of neural activity in the same brain regions. U50488-induced CPA and Fos levels were compared between knockin (KI) mice that lack pMeCP2-S421 and their wild-type (WT) littermates. U50488 administration acutely induced pMeCP2-S421 and Fos selectively in the NAc but did not alter MeCP2 levels in any brain region. U50488-induced CPA was associated with decreased pMeCP2-S421 in the ILC and BLA and induced Fos in the BLA. MeCP2 KI mice showed CPA indistinguishable from their WT littermates, but they also showed less BLA Fos induction upon CPA. These data are the first to show that pMeCP2-S421 is induced in the brain acutely after U50488 administration but not upon U50488-induced CPA. Although pMeCP2-S421 is not required for U50488-induced CPA, this phosphorylation event may contribute to molecular plasticities in brain regions that govern aversive behaviors.

The role of the insular cortex in naloxone-induced conditioned place aversion in morphine-dependent mice.

A negative emotional state resulting from the withdrawal of drug addiction is thought to be an important factor that triggers and exacerbates relapse. Since the insular cortex is a key brain structure involved in the modulation of negative emotions, we investigated whether the integrity of the insular cortex was important for motivational aversion associated with morphine withdrawal as well as whether this kind of negative emotion induced neuroadaptation in the insular cortex. In this present study, a sensitive mouse conditioned place aversion (CPA) model measuring the motivational aversion of morphine withdrawal was first established. Our results showed that bilateral insular cortex lesions by kainic acid completely inhibited the expression of CPA. The expression of FosB/deltaFosB in the insular cortex was significantly increased 24 h after the CPA regime was performed, but the expression of c-Fos in the insular cortex did not changed. These findings indicate that the integrity of the insular cortex is essential to motivational aversion associated with morphine withdrawal, and that this kind of aversion induces neuroadaptation, observed as the increase of FosB/deltaFosB expression, in the insular cortex.

Sex differences between CRF1 receptor deficient mice following naloxone-precipitated morphine withdrawal in a conditioned place aversion paradigm: implication of HPA axis.

BackgroundExtinction period of positive affective memory of drug taking and negative affective memory of drug withdrawal, as well as the different response of men and women might be important for the clinical treatment of drug addiction. We investigate the role of corticotropin releasing factor receptor type one (CRF1R) and the different response of male and female mice in the expression and extinction of the aversive memory.Methodology/Principal FindingWe used genetically engineered male and female mice lacking functional CRF1R. The animals were rendered dependent on morphine by intraperitoneally injection of increasing doses of morphine (10–60 mg/kg). Negative state associated with naloxone (1 mg/kg s.c.)-precipitated morphine withdrawal was examined by using conditioned place aversion (CPA) paradigm. No sex differences for CPA expression were found in wild-type (n = 29) or CRF1R knockout (KO) mice (n = 29). However, CRF1R KO mice presented less aversion score than wild-type mice, suggesting that CRF1R KO mice were less responsive than wild-type to continuous associations between drug administration and environmental stimuli. In addition, CPA extinction was delayed in wild-type and CRF1R KO male mice compared with females of both genotypes. The genetic disruption of the CRF1R pathway decreased the period of extinction in males and females suggesting that CRF/CRF1R is implicated in the duration of aversive memory. Our results also showed that the increase in adrenocorticotropic hormone (ACTH) levels observed in wild-type (n = 11) mice after CPA expression, were attenuated in CRF1R KO mice (n = 10). In addition, ACTH returned to the baseline levels in males and females once CPA extinction was finished.Conclusion/SignificanceThese results suggest that, at least, CPA expression is partially due to an increase in plasma ACTH levels, through activation of CRF1R, which can return when CPA extinction is finished.

Neuropeptide FF attenuates the acquisition and the expression of conditioned place aversion to endomorphin-2 in mice

It has been demonstrated that the endogenous mu opioid (MOP) agonist endomorphin-2 (EM-2) produces conditioned place aversion (CPA) and in contrast, morphine exerts opposite action. Neuropeptide FF (NPFF) was reported to act as a functional antagonist of mu opioid receptor and to exert opioid-modulating activities. The present study examined the influence of NPFF on the rewarding action of EM-2, using the unbiased conditioned place preference (CPP) paradigm. For testing the effect of NPFF on the acquisition of EM-2-induced CPA, NPFF and EM-2 were co-injected on the conditioning days without drug treatment on the followed test day. To explore the effect of NPFF on the expression of EM-2-induced CPA, EM-2 was administered alone on the conditioning days, and NPFF was given 5min before placement in the CPP apparatus on the test day. The results showed that NPFF (2.5, 5 and 10nmol, i.c.v.) alone caused little place preference change. However, NPFF dose-dependently reversed the acquisition of CPA induced by 30nmol EM-2 (i.c.v.). Similarly, the expression of EM-2-induced CPA was also reduced by NPFF. Moreover, the effects of NPFF on the acquisition and the expression of EM-2-induced CPA were completely blocked by the NPFF receptors antagonist RF9 (10nmol, i.c.v.). However, central injection of NPFF neither changed the locomotor activity nor modified the locomotor action of EM-2. These data provide the first evidence for a functional interaction of the endogenous ligands for NPFF and MOP receptors, and further support an anti-opioid character of NPFF system.

Role of IKK/NF-κB signaling in extinction of conditioned place aversion memory in rats.

The inhibitor κB protein kinase/nuclear factor κB (IKK/NF-κB) signaling pathway is critical for synaptic plasticity. However, the role of IKK/NF-κB in drug withdrawal-associated conditioned place aversion (CPA) memory is unknown. Here, we showed that inhibition of IKK/NF-κB by sulphasalazine (SSZ; 10 mM, i.c.v.) selectively blocked the extinction but not acquisition or expression of morphine-induced CPA in rats. The blockade of CPA extinction induced by SSZ was abolished by sodium butyrate, an inhibitor of histone deacetylase. Thus, the IKK/NF-κB signaling pathway might play a critical role in the extinction of morphine-induced CPA in rats and might be a potential pharmacotherapy target for opiate addiction.

The MEK inhibitor SL327 blocks acquisition but not expression of lithium-induced conditioned place aversion: a behavioral and immunohistochemical study

Recent evidence involves extracellular signal-regulated kinase (ERK) in positive motivational properties of drugs as determined by conditioned place preference but, to date, its role in conditioned place aversion (CPA) still awaits to be fully characterized. The aim of this study was to assess whether activated ERK (pERK) plays a role in the acquisition and/or expression of lithium-induced CPA. C57BL/6J mice were subjected to lithium (150 mg/kg)-induced CPA. The role of pERK was determined by administering the mitogen-activating extracellular kinase inhibitor, SL327, (a) 25 and 50 mg/kg, before each exposure to the lithium-associated compartment (acquisition), and (b) 25, 50, and 100 mg/kg, before post-conditioning test (expression). To assess whether ERK is activated by acute lithium and, in distinct experiments, during CPA expression, mice were sacrificed, 30 min after lithium, and immediately after post-conditioning test, respectively, for pERK immunohistochemistry. Lithium increased pERK-positive neurons in bed nucleus of stria termialis, in central and basolateral amygdala and elicited significant CPA. SL327 (50 mg/kg) significantly prevented its acquisition. In addition, the post-conditioning test of lithium-conditioned mice determined a significant increase of pERK-positive neurons in the dorsal striatum and SL327 (50 mg/kg), administered before post-conditioning test, while failing at the doses of 25, 50, and 100 mg/kg, to affect lithium-induced CPA expression, completely prevented it. These results indicate that pERK is critical for acquisition, but not expression, of lithium-induced CPA and that its activation in the dorsal striatum, during expression, is not critical for retrieval of the aversive memory.

Orexins in the midline thalamus are involved in the expression of conditioned place aversion to morphine withdrawal

Previous studies have implicated the bed nucleus of the stria terminalis, central nucleus of the amygdala and the shell of the nucleus accumbens (collectively called the extended amygdala) as playing an important role in mediating the aversive emotion associated with opioid withdrawal. The paraventricular nucleus of the thalamus (PVT) provides a very dense input to the extended amygdala, and the PVT is densely innervated by orexin neurons, which appear to be involved in producing some of the physical and emotional effects associated with morphine withdrawal. In the present study, we confirm that the PVT is densely innervated by orexin fibers, whereas the regions of the extended amygdala associated with the effects of morphine withdrawal are poorly innervated. Microinjections of the orexin-1 receptor (OX1R) antagonist SB334867 or the orexin-2 receptor (OX2R) antagonist TCSOX229 at doses of 5.0 or 15.0 microg into the PVT region did not affect the acquisition of the conditioned place aversion (CPA) nor the physical effects produced by naloxone-precipitated morphine withdrawal. In contrast, microinjections of TCSOX229 (15.0 microg) in the PVT region significantly attenuated the expression of naloxone-induced CPA while microinjections of SB334867 at the same dose had no effect. The results from these experiments indicate a role for OX2R in the PVT on the expression of CPA associated with morphine withdrawal. Orexins may mediate the aversive effects of morphine withdrawal by engaging the extended amygdala indirectly through the action of orexins on the PVT.

Pentylenetetrazol produces a state-dependent conditioned place aversion to alcohol withdrawal in mice

The purpose of this study was to determine if aversive effects of alcohol withdrawal could be detected in mice using the place conditioning procedure and whether the GABA A receptor antagonist, pentylenetetrazol (PTZ), would increase the aversive effects of alcohol withdrawal and increase the probability of detecting conditioned place aversion. Subjects were alcohol-naïve mice from a specific line selectively bred for low alcohol preference (LAP1; n = 91) and were assigned to three groups: alcohol withdrawal, PTZ alone, and PTZ + alcohol withdrawal. On four trials, mice received either a 4.0 g/kg intraperitoneal (i.p.) injection of alcohol (alcohol withdrawal, PTZ + alcohol withdrawal groups) or saline (PTZ group) 8 h prior to being placed on a distinctive floor texture for a 30-min conditioning session. Five minutes before these sessions, mice in the PTZ and PTZ + alcohol withdrawal groups received PTZ (5.0 mg/kg; i.p.) and the alcohol withdrawal group received saline. On intervening days mice received two saline injections at the same time points prior to being placed on a different floor texture. Post-conditioning floor preference was assessed in two 60-min tests; the first test was drug-free and the second test was state-dependent. Neither alcohol withdrawal nor PTZ produced significant place conditioning. The PTZ + alcohol withdrawal group showed a significant place aversion during the state-dependent test. These data suggest that the combined stimulus properties of PTZ and alcohol withdrawal facilitated the expression of conditioned place aversion to alcohol withdrawal.

Glutamate receptors in the dorsal hippocampus mediate the acquisition, but not the expression, of conditioned place aversion induced by acute morphine withdrawal in rats.

To evaluate the role of glutamate receptors in the dorsal hippocampus (DH) in the motivational component of morphine withdrawal. NMDA receptor antagonist D-AP5 (5 microg/0.5 microL per side) or AMPA receptor antagonist NBQX (2 microg/0.5 microL per side) was microinjected into DH of rats. Conditioned place aversion (CPA) induced by naloxone-precipitated morphine withdrawal were assessed. Preconditioning microinjection of D-AP5 or NBQX into the DH impaired the acquisition of CPA in acute morphine-dependent rats. However, intra-DH microinjection of D-AP5 or NBQX after conditioning but before the testing session had no effect on the expression of CPA. Our results suggest that NMDA and AMPA receptors in the dorsal hippocampus are involved in the acquisition, but not in the expression, of the negative motivational components of acute morphine withdrawal in rats.

Expression of c-Fos in the rat central amygdala accompanies the acquisition but not expression of conditioned place aversion induced by withdrawal from acute morphine dependence

Conditioned reinforcement is hypothesized to be critically involved in drug addiction as a factor contributing to compulsive drug use and relapse. The present study focused on the neurobiology involved in the acquisition and expression of conditioned reinforcing effects of morphine withdrawal employing a conditioned place aversion (CPA) paradigm in acute-dependent rats. Expression of c-Fos in the amygdala (the central nucleus, CeA; the medial nucleus, MeA; the basolateral nucleus, BLA) following naloxone-precipitated withdrawal and the CPA test was examined using a range of naloxone doses (0.02, 0.05, 0.1, 0.2, 0.5 and 1.0 mg/kg). Naloxone dose-dependently produced CPA in rats given a single morphine exposure. In CeA, but not MeA with high-level constitutive neuronal activity, the naloxone-induced modification in c-Fos immunoreactivity following morphine pretreatment exhibited a dose-dependent pattern similar to that seen in the behavioral study. On the other hand, none of the three amygdaloid nuclei examined including CeA, MeA and BLA showed notable sensitivity of c-Fos to the conditioned withdrawal stimulus. These results suggest that CeA may play a role in the negative affective aspect of withdrawal from acute dependence, and in part suggest that the acquisition and expression of CPA may involve different neurobiological mechanisms.

Handling blocks expression of conditioned place aversion but not conditioned place preference produced by ethanol in mice

Previous findings implicate opioid receptors in the expression of the conditioned rewarding and aversive properties of ethanol. We have recently reported that the conditioned rewarding effect of ethanol is mediated by opioid receptors in the ventral tegmental area (VTA). We attempted to determine whether VTA opioid receptors also mediate the expression of the conditioned aversive properties of ethanol. However, the magnitude of conditioned place aversion (CPA) was not consistent with our previous findings and prevented us from making definitive conclusions. We hypothesized that the handling required to make intracranial infusions in mice alters the expression of CPA, but not conditioned place preference (CPP). Therefore, non-operated animals underwent a Pavlovian conditioning procedure for either ethanol CPA or CPP. Just before testing, half of the animals were held by the scruff of the neck to mimic intracranial infusion handling. Animals conditioned for CPA did not express CPA if they were handled. However, animals conditioned for CPP exhibited robust CPP, regardless of handling. These findings provide additional evidence that the conditioned rewarding and aversive effects of ethanol are mediated by different neural mechanisms.

Glutamatergic activation of anterior cingulate cortex produces an aversive teaching signal.

Noxious stimuli have motivational power and can support associative learning, but the neural circuitry mediating such avoidance learning is poorly understood. The anterior cingulate cortex (ACC) is implicated in the affective response to noxious stimuli and the motivational properties of conditioned stimuli that predict noxious stimulation. Using conditioned place aversion (CPA) in rats, we found that excitatory amino acid microinjection into the ACC during conditioning produces avoidance learning in the absence of a peripheral noxious stimulus. Furthermore, microinjection of an excitatory amino acid antagonist into the ACC during conditioning blocked learning elicited by a noxious stimulus. ACC lesions made after conditioning did not impair expression of CPA. Thus, ACC neuronal activity is necessary and sufficient for noxious stimuli to produce an aversive teaching signal. Our results support the idea that a shared ACC pathway mediates both pain-induced negative affect and a nociceptor-driven aversive teaching signal.

Diazepam in the ventral striatum dissociates dopamine-dependent and dopamine-independent place conditioning.

We have previously shown that diazepam blocks both the formation and the expression of amphetamine-induced conditioned place preference (CPP), but has no effect on the CPP induced by morphine. Because diazepam reduces dopamine activity in the nucleus accumbens, three experiments were conducted in order to investigate whether diazepam selectively blocks the expression of place conditioning dependent on mesolimbic dopamine. The first experiment found that systemic diazepam blocked the expression of conditioned place aversion induced by the kappa-receptor agonist U50-488. The second and third experiments demonstrated that intra-cranial injections of diazepam in the nucleus accumbens blocked the expression of amphetamine CPP but not of morphine CPP. It is concluded that diazepam interferes with mesolimbic dopamine-dependent motivational effects of drugs.

Extinction of ethanol-induced conditioned place preference and conditioned place aversion: effects of naloxone.

Four experiments examined the effect of naloxone pretreatment on the expression and extinction of ethanol-induced conditioned place preference (experiments 1, 2, 4) or conditioned place aversion (experiments 1, 3). DBA/2 J mice received four pairings of a distinctive tactile (floor) stimulus (CS) with injection of ethanol (2 g/kg) given either immediately before or after 5-min exposure to the CS. A different stimulus was paired with injection of saline. Pre-CS injection of ethanol produced conditioned place preference, whereas post-CS injection of ethanol produced conditioned place aversion. Both behaviors extinguished partially during repeated choice testing after vehicle injection. Naloxone (10 mg/kg) had little effect on the initial expression of conditioned place preference, but facilitated its extinction. Moreover, repeated naloxone testing resulted in the expression of a weak conditioned place aversion to the CS that initially elicited a place preference. In contrast, naloxone (1.5 or 10 mg/kg) enhanced expression of conditioned place aversion, thereby increasing its resistance to extinction. A control experiment (experiment 4) indicated that repeated testing with a different aversive drug, lithium chloride, did not affect rate of extinction or produce an aversion to the CS previously paired with ethanol. These findings do not support the suggestion that naloxone facilitates the general processes that underlie extinction of associative learning. Also, these data are not readily explained by the conditioning of place aversion at the time of testing. Rather, naloxone's effects appear to reflect a selective influence on maintenance of ethanol's conditioned rewarding effect, an effect that may be mediated by release of endogenous opioids. Overall, these findings encourage further consideration of the use of opiate antagonists in the treatment of alcoholism.

The Effects of Naloxone on Expression and Acquisition of Ethanol Place Conditioning in Rats

Naloxone has been shown to facilitate extinction of ethanol-induced conditioned place preference (CPP) in mice. The present-study extended these findings by examining naloxone’s effect on the expression (Experiment 1) and acquisition (Experiment 2) of place conditioning with ethanol in rats. In Experiment 1, after place conditioning with ethanol (1.8 g/kg, IP), groups N0, N1.5, and N10 received 0, 1.5, or 10 mg/kg naloxone before testing. As expected, ethanol produced a robust conditioned place aversion (CPA). However, naloxone had no effect on expression of CPA. In contrast to studies with mice, the endogenous opioid system does not appear to be involved in the conditioned motivational effects of ethanol in rats. In Experiment 2, groups SE1 and SE2, NS(1.5), NE(1.5), and NE(10), received ethanol alone (1.2 g/kg), naloxone alone (1.5 mg/kg), naloxone 1.5 mg/kg plus ethanol, and naloxone 10 mg/kg plus ethanol during acquisition, respectively. All naloxone-treated groups exhibited CPA. Moreover, group NE(1.5) showed a stronger CPA than group NS(1.5). The CPA produced by coadministration of naloxone and ethanol was attributed to naloxone’s effects on the neural processes underlying ethanol’s unconditioned aversive effects, or to other nonspecific effects on ethanol’s motivational properties.